Honestly, they will not resell a modified car at the OEM dealer level. Not at that modification level with the emission controls removed and all. What they will do is call up the auction house or consult the latest data sheets and see how much that car is worth as a 2008 STi. It will go to auction, I can almost assure you of that. The condition of it will play little to no role in the value they assess it at. The main number will come from "what does a 2008 STi bring at auction".
So yes, private party is your best bet. If they happen to give you a good trade in on it, check those numbers well because I promise they will take it back from you somewhere else in the financing deal.